@article{ZNSL_2018_470_a5,
     author = {V. A. Koibaev},
     title = {On a~question about generalized congruence {subgroups.~I}},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {105--110},
     publisher = {mathdoc},
     volume = {470},
     year = {2018},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2018_470_a5/}
}